This invention relates to a method of densifying fumed metal oxides, and to the use of such densified oxides in forming slip cast articles.
A "fumed" metal oxide is a finely divided material, such as silica, that may be prepared by either a vapor phase oxidation or a combustion process. In a typical vapor phase process, vapors of a metal compound, such as silicon tetrachloride (SiCl.sub.4), are thermally converted in the presence of moisture to the corresponding oxide, e.g. silica. In a combustion process, a combustible metal compound, such as a silicon halide or halosilane, is burned to produce the oxide. The former process is described, for example, in U.S. Pat. No. 2,272,342 granted Feb. 10, 1942 to J. F. Hyde, and No. 2,268,589, granted Jan. 6, 1942 to J. A. Heany. The latter process is described, for example, in U.S. Pat. No. 2,823,982, granted Feb. 18, 1958 to O. Saladin et al.
The oxides thus obtained are conventionally finely divided, low density, fluffy materials characteristically having an average particle size in the range of 10 to 50 millimicrons and a surface area in the range of 100 to 400 m.sup.2 /gram. Typically, the particles are agglomerated in clusters reminiscent of grape clusters. Fumed SiO.sub.2, fumed TiO.sub.2, and fumed Al.sub.2 O.sub.3 of high purity are available commercially. Such oxide materials are widely used as additives in paints, rubber products, and other organic materials.
It has been recognized that such fumed oxides are highly active, and that glass-forming oxides can be consolidated to solid glass bodies below normal melting temperatures. The processes mentioned provide convenient means of obtaining high purity glass components by starting with high purity materials. Thus, the purity of the component, and hence the glass, is dependent on the purity of the materials processed. High purity glasses may be defined as containing no more than ten parts per million (ppm) alkali metal ions, less than one ppm transition metal ions, and less than twenty ppm total metal impurities, that is, metals exclusive of any intentionally added dopants.
One need for such high purity glasses occurs in melting crucibles used by the semiconductor industry. In particular, high purity silica crucibles are desired for melting and drawing silicon. Such crucibles may be produced by silica deposition on a mandrel to form a preform, followed by thermal consolidation of the preform as described, for example, in U.S. Pat. No. 3,741,796, granted June 26, 1973 to Jack Walker. Alternatively, such crucibles have been produced by comminuting fused quartz, suspending the particles in a casting medium, and forming the crucible in a plaster mold by conventional slip casting practice. This procedure, while widely practiced, has the drawback that most quartz raw materials do not meet high purity standards without purification, as by leaching.
It would therefore be desirable to use fumed oxides, especially fumed silica, to form shaped bodies such as crucibles. However, it is very difficult to form clear, homogeneous bodies by vitrification of green bodies compressed from these fluffy materials by ordinary pressing or pelletizing techniques. Furthermore, they do not lend themselves, in their low density form, to mixing with the vehicles normally used for slip casting. It is therefore a primary purpose of this invention to circumvent these problems by providing a method of densifying fumed metal oxides, especially silica, whereby such oxides can be successfully employed in the production of hot pressed or slip cast articles, as well as for other purposes.